Making deep power diodes

ABSTRACT

A process for forming aluminum doped silicon semiconductor material for large area semiconductor devices embodies thermal gradient zone melting processing and migration of a molten zone of a predetermined thickness to assure stability of the molten zone while migrating.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This invention is a Continuation-in-Part of our patent application Ser.No. 519,249, filed Oct. 30, 1974, and now U.S. Pat. No. 3.956,023, thesame being a Continuation-in-Part of our patent application, Ser. No.411,001, filed Oct. 30, 1973, now abandoned, and both of which areassigned to the same assignee as the present invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to large area semiconductor power diodes and atemperature gradient zone melting process for making the same.

2. Description of the Prior Art

Heretofore, commercial seimconductor diodes have been made by diffusion,alloying or epitaxial growth techniques. All such processes involveprocedures wherein the optimum physical characteristics of the diodesare never achieved. Procedures are such that the introduction ofcontaminants inherent with the process techniques practiced degrade thephysical characteristics of the diodes manufactured.

W. G. Pfann in his U.S. Pat. No. 2,813,048, teaches thermal gradientzone melting techniques for making various semiconductor devices.However, Pfann's devices had too many deficiencies to enable the processto be employed commercially.

It is desirable to have large area power diodes made from commerciallyavailable semiconductor wafers provided a reliable efficient methodcould be developed to make the diodes.

An object of this invention is to provide a new and improved process formaking a large area semiconductor power device, the structure of whichovercomes the deficiencies of the prior art.

Another object of this invention is to provide a new and improvedprocess for making a large area semiconductor power diode which hasmaximum theoretical breakdown voltage as determined by the materialsembodied therein.

Another object of this invention is to provide a new and improvedprocess for making a large area semiconductor power device embodying asharper P-N junction profile than any P-N junctions in prior artdevices.

A further object of this invention is to provide a new and improvedprocess for making a large area semiconductor power diode which has afaster recovery time and better switching characteristics than the priorart power diode devices.

A still further object of this invention is to provide a new andimproved temperature gradient zone melting process to make new andimproved large area semiconductor power devices which include regions ofrecrystallized semiconductor material which are substantially free ofdefects such, for example, as metallic inclusions.

Other objects of this invention will, in part, be obvious and will, inpart, appear hereinafter.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the teachings of this invention, there is provided alarge area semiconductor power diode comprising at least two integralregions of semiconductor material. Each region has a predetermined levelof resistivity and two major opposed surfaces which are, respectively,the top and bottom surfaces thereof. Each of the at least two regions isproduced from an individual wafer, or body, of semiconductor material.The wafers are oriented in a manner whereby the major surface of one isin an abutting contact relationship with each other. A layer of metalvapor deposited on at least one of the abutting surfaces is migratedthrough the wafer, on whose major surface it is deposited, by thermalgradient zone melting to physically join the two wafers together and toform a region of recrystallized semiconductor material of that wafer.The region of recrystallized semiconductor material is formed in situ bythe migration of a melt of metal-rich semiconductor material along athermal gradient aligned parallel with a first preferred crystal axis ofthe material of one wafer by a thermal gradient zone melting processpracticed at a predetermined elevated temperature. The predeterminedlevel of resistivity of the recrystallized material is determined by thesolid solubility limit of that metal migrated through that wafer in thatsemiconductor material of the wafer at that predetermined elevatedtemperature. The resulting structure is two integral regions, each of apredetermined type conductivity. When the regions are of opposite typeconductivity, a P-N junction is formed therebetween.

DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are elevation views, in crosssection, of bodies ofsemiconductor material being processed in accordance with the teachingsof this invention;

FIG. 3 is a schematic of the relationship of the thickness of a melt ofmetal-rich semiconductor material migrating through a solid body ofsemiconductor material by TGZM at an elevated temperature;

FIG. 4 is an elevation view, partly in crosssection, of apparatussuitable for practicing the process of this invention;

FIG. 5, 6, 7, 8 and 9 are elevation views, in cross-section, of thebodies of semiconductor material of FIGS. 1 and 2 being processedfurther in accordance with the teachings of this invention;

FIG. 10 is a graph of reverse current in amperes vs. reverse bia involts of a semiconductor power diode made in accordance with theteachings of this invention; and

FIGS. 11, 12 and 13 are elevation views of a body of semiconductormaterial being processed by an alternate method in accordance with theteachings of this invention.

DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is shown a first body 10 of singlecrystal semiconductor material having a first type conductivity and apreferred resistivity. The body 10 has a top surface 12 and a bottomsurface 14 which are two major surfaces substantially parallel to eachother.

The body 10, as referred to by us, is, by our definition a wafer ofsemiconductor material normally employed in the semiconductor industryand avaiable on the commercial market. Such wafers are of the order of 1inch, 11/2 inch, and up to 3 or more inches in diameter. The wafers areat least 4 mils in thickness to enable workers to handle the wafersproperly without incurring excessive losses resulting from breakage ofthe wafers during process handling. Other wafers of thicknesses of 6, 8,10 mils and more are also suitable. Alternatively, one may also cut hisown wafers or bodies 10 from commercially available rod stock or fromin-plant grown material. In such instances, the thickness of the waferor body 10 may exceed 10 mils and be as thick as desired for the finalproduct and which can be reasonably processed.

The resistivity of the material comprising the body 10 varies with therequirement for the desired breakdown voltage of the device to be built.The body 10 has a preferred planar (111) crystallographic structure forthe orientation of its surfaces 12 and 14. This preferred (111) planarcrystallographic structure of the surfaces is required to enable theliquid metal to maintain a substantially uniform front which advancesforward through the body 10 during a temperature gradient zone meltingprocess employed in this invention. It has been discovered that themigration of the liquid melt, that is the metal composition ofsemiconductor material and the material deposited on the surface, mustbe in a direction normal to the semiconductor metal liquid system. Thus,the planar zone is bounded by the (111) facet plane. If the migrationproceeds oblique to the (111) facet plane, then the liquid melt zonebecomes unstable and breaks up into droplets. Any droplet which becomeentrapped in the body 10 upon completion of migration results in defectssuch, for example, as stressed solid metallic inclusions, which affectthe physical and operating characteristics of the diode by inducingstrain and stresses in body 10. Any objects imbedded in the P-N junctionresults in current leakage and degradation of the electricalcharacteristics of the device. Suitable materials for comprising thebody 10 are silicon, silicon carbide, gallium arsenide, germaniumcompounds of a Group II element and a Group VI element and compounds ofa Group III element and Group V element.

In order to describe the invention more fully, the body 10 will bedescribed as being of silicon semiconductor material having a thicknessof about 10 mils and a diameter of approximately 11/2 inches.

The body 10 of silicon is prepared in the customary way of allsemiconductor materials for metal vapor deposition by such suitableprocess means as grinding, polishing, lapping, and chemical polishing toremove damage layers of semiconductor material. The process of preparingthe body 10, including the aforementioned process steps, which are notshown, is not described in detail for such processing and treatment ofthe body 10 is well known in the art and is not pertinent to theinvention herein.

A layer of a suitable metal is deposited on the top surface 12 of thebody, or wafer, 10. The material of the metal layer 16 is one that willimpart to the material of the body 10 a second and opposite typeconductivity, intrinsic conductivity, or the same type conductivity asbefore but of a different level of resistivity. The material of themetal layer 16 may also include two or more different materials in orderto tailor the level of resistivity of the recrystallized semiconductormaterial to be produced by the thermal gradient zone melting process.The thickness of the layer 16 is such that when the material of thelayer 16 melts and forms a melt, or molten zone, of metal-richsemiconductor material, the thickness of the melt, or molten zone, willnot exceed the limits established for the stable migration of the moltenzone through the solid semiconductor material.

The thickness of the melt of metal-rich semiconductor material to bemigrated through the body, or wafer, 10 is temperature dependent. Wehave discovered that there is a critical thickness L which is dependentupon the temperature at which the thermal gradient zone melting processis practiced.

Referring now to FIG. 3, there is illustrated a schematic of therelationship of a melt of metal-rich semiconductor material of thicknessL migrating through a solid body of semiconductor material by TGZM at anelevated temperature T₁. L is the critical thickness of the melt zone ofa metal-rich semiconductor material to be migrated through the body 10from the colder surface toward the hotter surface. The front face of themelt zone is at temperature T₁ and the rear surface of the melt zone isat T₂. The thermal gradient across the thickness L of the melt zone isG_(L). If K_(S) is the thermal conductivity in the solid material andK_(L) is the thermal conductivity in the liquid, then

     G.sub.L =  K.sub.S /K.sub.L G.sub.S,                      (1)

where

G_(S) is the thermal gradient in the solid material.

The dotted line connecting C_(EC) - C_(EH) represents the drop in theequilibrium, or theoretical, concentration of the semiconductor materialthrough the melt, or molten zone, at zero velocity of the migratingliquid zone. However, since the molten zone is moving at a givenvelocity V, expressed in centimeters per second (cm/sec), the slope ofthe solid line C_(C) - C_(H), which is the actual drop in theconcentration of the semiconductor material in the liquid of the moltenzone, deviates from the theoretical line C_(EC) - C_(HC). The value forU, which is the undersaturation of the semiconductor material in theliquid of the molten zone expressed in atomic percent of thesemiconductor material required at the hot forward interface of themolten zone and the solid original material, is increased. At the sametime the value of S, the supersaturation of the semiconductor materialin the liquid of the molten zone at the colder rear interface, expressedin atomic percent of semiconductor material, is required to drive thedeposition, or crystallization, process of semiconductor material fromthe melt, or molten zone. The semiconductor material is dissolved at thehot forward interface, diffuses through the molten zone toward thecolder rear interface where it is deposited as the recrystallizedsemiconductor material. The recrystallized semiconductor material has apredetermined concentration of the metal of the molten zone, distributedsubstantially uniformly throughout, as determined by the solidsolubility limit of that metal in that semiconductor material at therear interface temperature T₂. Since the temperature T₂ at the rearinterface is slightly less than the temperature T₁, at the forwardinterface, T₂ is generally referred to be the same as T₁, forsimplification. This difference in values between T₁ and T₂ appears tohave no appreciable effect on results obtained since the difference isless than the order of a degree.

The change in concentration of dissolved semiconductor material in themetal-rich semiconductor liquid material with temperature can beexpressed by the mathematical relationship

    (dC/dT)                                                    (2)

if G_(L) is the thermal gradient in the liquid (molten zone) and (dC/dT)is the change in concentration of dissolved semiconductor material inthe metal-rich liquid with temperature, then in order for the moltenzone of L thickness to be stable during migration, the zone thickness L,we have discovered, may be expressed as follows: ##EQU1##

When L has a value which is greater than that determined by equation(3), we have found that the liquid, or molten zone, becomes unstableduring migration and begins to break up into a plurality of liquidregions.

The value of L may also be expressed in terms of a constant A, which isfor a specific metal-semiconductor material system, and the migrationvelocity, V, of the molten zone expressed in centimeters per degreesCelrius. Expressed in these terms, equation (3) becomes ##EQU2##

We have at this time worked primarily with the aluminum-silicon system.From our work we have prepared the following Table which tabulates thedata we obtained by practicing the migration of aluminum, andpredominantly aluminum metal systems, through silicon semiconductormaterial at various temperatures of migration by a thermal gradient zonemelting process.

                  TABLE                                                           ______________________________________                                         Temp                                                                                ##STR1##                                                                                    ##STR2##      L                                          (°C)                                                                         (cm.sup.2 /°C-sec)                                                                   (atomic %/°C)                                                                        (microns)                                   ______________________________________                                         600  7 × 10.sup.-8                                                                         6.6 × 10.sup.-4                                                                       212                                          700  8 × 15.sup.-8                                                                         7.5 × 10.sup.-4                                                                       1213                                         800  10.sup.-7     7.9 × 10.sup.-4                                                                       253                                          900  1.2 × 10.sup.-7                                                                       8.4 × 10.sup.-4                                                                       286                                         1000  1.7 × 10.sup.-7                                                                       8.8 × 10.sup.-4                                                                       386                                         1100  2.2 × 10.sup.-7                                                                       10 × 10.sup.-4                                                                        440                                         1200  5 × 10.sup.-7                                                                         12.1 × 10.sup.-4                                                                      826                                         1300  10.sup.-6     14.3 × 10.sup.-4                                                                      1400                                        1400  3 × 10.sup.-6                                                                         16.6 × 10.sup.-4                                                                      3600                                        ______________________________________                                    

The above tabulated data is suitable for determining L even when up to10 percent of the metal migrated consists of one or more other dopants,or intrinsic, materials, relative to silicon, in the aluminum metal.

For the aluminum-silicon system, we have found that (U + S) is equal to200 atomic percent of silicon per centimeter per second times thevelocity of the molten zone of aluminum-rich silicon through a solidbody of silicon. Substituting for (U + S) in equation (4) for analuminum-silicon system we have the following: ##EQU3##

As an example of the use of the data of the Table we can find the valuefor L_(Al-Si) at 1100° C for the migration temperature and a temperaturegradient, G_(L), of 50° C per centimeter as follows: ##EQU4##

For stable migration of the molten zone, its width, or thickness, L,should be less than 440 microns.

However, we have found that the smaller the value of L is retained, theless one has to worry about maintaining the thermal gradient alignedsubstantially with the preferred crystal axis of <111> for the materialof the wafer. A slight deviation of up to 3° or 4° is permissible.Further, less metal material is required for migration through thewafer. As described later, we prefer a value of 10 to 20 microns for L.

However, one must have good wetting of the surface by the metal tomigrate in order to initiate a stable molten zone. Foils and sheetsapparently do not provide the intimate contact required between the twomaterials.

In order to describe the invention more fully, the semiconductormaterial is said to be of N-type conductivity silicon and the metal ofthe layer 16 to be of aluminum to produce the second, and opposite typeconductivity, P-type conductivity, in the body 10. The layer 16 ofaluminum is deposited by any suitable means known to those skilled inthe art such, for example, hot filament vacuum deposition, electron-beamdeposition and the like, which will produce a substantially oxygen-freedeposit of metal for the temperature gradient zone melting process to bepracticed. The metal layer must be uniformly defect-free and clean toavoid wetting problems and inclusions which result in unstablemigration. A deposited thin film is superior to metal foil materialbecause with a deposited film a clean, adherent, intimate and uniformcontact to the semiconductor body is formed that is free of foreigninclusions, bubbles and areas of noncontact.

The layer 16 of metal must be thick enough to cover the entire surface12 with metal. Normally, a thickness of no less than 1 micron has beendiscovered to be adequate to assure complete coverage of the surface 12.However, the layer 16 should not be greater than approximately L micronsin thickness. It has been discovered that when the layer 16 exceeds Lmicrons in thickness, the melt zone which is migrated through the body10 also exceeds L microns in thickness. When the thickness of the meltzone exceeds L microns, the melt zone encounters intrinsic instabilitycreated by thermal and mass transport conditions and the zone becomesunstable at the solidifying solid-liquid interface.

With reference to FIG. 2, a second body, or wafer, 18 of silicon singlecrystal semiconductor material of N-type conductivity and a selectedresistivity is prepared in the same manner as the first body 10. Thebody, or wafer, 18 is defined in the same manner as previously definedfor the body, or wafer. The second body 18 has a top surface 20 and abottom surface 22 which are two major surfaces substantially parallel toeach other. The second body 18 of silicon may have any of thecrystallographic structures which are inherent of silicon material.Preferably, however, to avoid confusion in making a large area powerdiode, the second body 18 of silicon also has a (111) planarcrystallographic structure for the surfaces 20 and 22. It is to be notedthat body 18 is preferably of the same material as the body 10.

A sandwich of the two processed silicon bodies 10 and 18 is prepared.The first body 10 is disposed on the second body 18 and so oriented thatthe exposed major surface 24 of the layer 16 of aluminum is disposed on,and in physical contact with, the top surface 20 of the second body 18of silicon.

The prepared sandwich is placed in an apparatus suitable for producing athermal gradient substantially coinciding with the vertical axis of thebodies 10 and 18 between the two surfaces 14 and 22. The thermalgradient is therefore substantially aligned with the <111> axis of thebody 10. The bottom surface 14 of the first body 10 of silicon isoriented so as to be exposed to a source of thermal energy wherein thetemperature is higher than that to which the bottom surface 22 of thebody 18 of silicon is exposed. A temperature gradient zone melting(TGZM) process is practiced to produce a large area power diode. Atemperature gradient of from 10° C per centimeter to 200° C percentimeter is maintained in the bulk of silicon material during themigration of the layer 16 of aluminum through the body 10. Preferably,the temperature gradient is maintained at a calculated temperaturegradient of 100° C/cm in the bulk of silicon comprising the sandwich.The calculated temperature gradient ∇T_(s) is derived from the equation:

    K.sub.S ∇ T.sub.s = E γ T.sub.c.sup.4

where

K_(s) is the thermal conductivity of the semiconductor material-silicon.

E is the emissivity of the semiconductor material-silicon.

γ is the Stefan-Boltzman Radiation constant.

T_(c) is the temperature of the cold face of the body of semiconductormaterial-silicon.

∇T_(s) is the temperature gradient in the body of the semiconductormaterial.

The temperature gradient zone melting (TGZM) process may be carried outat a pressure ranging from approximately 1 × 10⁻⁷ torr to approximately1 × 10⁻⁵ torr. Alternatively, the process may also be practiced atatmospheric pressure embodying a suitable source of radiant energy.

A schematic of a suitable vacuum furnace apparatus 50 for thetemperature gradient zone melting process of this invention is shown inFIG. 4. The processed sandwich of FIG. 2 is disposed in chamber 15 ofapparatus 50 between a hot source 52 and a cold source 54. The hotsource may, for example, be a block of molybdenum suitably heated by asource 56 of thermal energy such, for example, as an electron beam. Itis essential that the hot source 52 uniformly heat the bottom surface 14of the body 10 to a temperature of from 700° C to 1400° C to achieve therange of calculated temperature gradient of from 50° C per centimeter to200° C per centimeter. The cold source 54 may, for example, be awater-cooled copper block. When the cold source 54 is a water-cooledblock maintained at from 25° C to 50° C and the top surface 14 of thebody 10 is uniformly heated to 1000° C ± 20° C, the calculatedtemperature gradient is 100° C per centimeter for the bulk of siliconmaterial through which the aluminum layer 16 is to be migrated.Radiation shields 58 are provided about the processed sandwich and thehot and cold sources 52 and 54, respectively, to prevent excessive lossof heat or possible influence on the thermal gradient in the bodies 10and 18 during the TGZM process. In particular, the radiation shieldsprevent thermal energy from being transported to or from the sides ofthe sandwich. The processed sandwich is disposed on supporting rib 31 ofthe radiation shields 58 in an aperture thereof defined by wall surface38.

Referring now to FIGS. 5 and 6 initially, upon being heated to theoperating temperature for the temperature gradient zone melting process,the layer 16 (FIG. 2) of aluminum becomes molten and alloys with theimmediately adjacent silicon material of the top surfaces 12 and 20 ofthe respective bodies 10 and 18 to produce a continuous molten interfaceof aluminum-rich silicon. The molten interface continues to increase insize until a pool of aluminum-rich silicon liquid 17, preferably nogreater than about L microns in thickness, is formed between the twobodies 10 and 18. As a result of the temperature gradient, the pool ofaluminum-rich silicon liquid 17 begins to migrate up through the body 10from the top surface 12 thereof towards the bottom surface 14.Aluminum-doped recrystallized silicon 30 of the body 10 and that of thetop surface 20 of the body 18 is deposited beginning at the interface ofthe layer 16 and the body 18 thereby creating a P-N junction 26. Themigration of the aluminum-rich silicon liquid upward through the body 10is accomplished by establishing and maintaining the aforementionedfinite thermal gradient in a first direction substantially parallel tothe vertical axis of the sandwich. Coextensively in time, a second orzero temperature gradient is established and maintained in the sandwichin a second direction which is normal to the first direction through theassistance of the radiation shields 58 of the apparatus 50. Theinterface 28 between the aluminum-rich silicon liquid 17 and the siliconof the body 10 is a large liquid area in the silicon body 10 having a(111) planar crystallographic structure and a <111> migration directionand migrates as a substantially continuous planar surface. Examinationof the front as it migrates up through the body reveals it to becontinuous throughout its expanse. As long as the thermal gradient ismaintained, the molten pool of aluminum-rich silicon 17 continues tomigrate upward through the body 10 continually creating a new moltenzone of aluminum and silicon at its front interface 28 and depositing analuminum doped recrystallized silicon region 30 of ever increasing size.The recrystallized region 30 has a solid solubility of the dopantimpurity therein which imparts the conductivity type and selected levelof resistivity to the region 30. Because the temperature gradient acrossthe thickness of the body 10 is small, and the slope of the solidsolubility curve for aluminum in silicon is small in the temperaturerange in which the temperature gradient zone melting is practiced, thedopant concentration and the resistivity throughout the recrystallizedregion 30 is substantially constant. That is, the impurity concentrationof aluminum in the silicon of the region 30 is approximately 2 × 10¹⁹atoms per cubic centimeter. An added benefit of this process is that themigration of the melt through the body provides a zone refining processfor the material of the body. As a result, the material is cleansed ofimpurities and some defects during the process.

In silicon having crystallographic structure orientation other than<111>, the molten interface 28 of the migrating aluminum-silicon melt isvery irregular. If the deviation from the <111> growth axis be toogreat, the migrating interface 28 breaks up and results in a highlyirregular P-N junction in the body 10 and a large area power diode ofundesirable electrical characteristics. However, when the migration ismaintained substantially along the <111> axis, the temperature gradientzone melting process continues with a substantially uniform molteninterface 28 until all of the body 10 is converted to an aluminum-dopedregion 30 of recrystallized silicon material of the body 10. The excessaluminum is removed from the surface 14 upon completion of the process.The completed large area power diode structure is illustrated in FIG. 6.

Referring now to FIG. 7, there is shown the breaking up of the melt zone17 which occurs when migration is not practiced substantially along the<111> axis of the body 10. As stated heretofore, the migration of thezone 17 must proceed normal to the natural solid facet plane of thesemiconductor metal liquid system. The planar melt zone 17 is thereforebonded by the facet plane. When migration proceeds oblique to the facetplane, the zone 17 breaks up into various sizes of droplets 19. Thelarger of the droplets 19 migrate faster than the next largest sizedroplets. Experience has shown that usually all of the larger droplets,ranging from 100 microns in diameter upward, are migrated out of thebody 10 during the process. The next largest group of droplets rangingin size from 100 to 10 microns in diameter do not migrate as fast andsome of the droplets 21 remain behind as metal inclusions uponcompletion of migration of the zone 17. However, droplets no larger thanabout 10 microns in diameter do not migrate, or migrate so slowly as tohave almost no movement, and consequently remain behind in the region 30as metal inclusions. These metal inclusions are defects which affect theoperating characteristics of the completed diode.

With reference to FIG. 8, there is shown a condition which occurs whenthe planar melt zone 17 is greater than about 20 microns in thickness.The front face 28 of the planar zone 17 remains intact. However, becauseof thermal gradient and mass transport of melt through the body 10, thezone 17 encounters intrinsic instability and the rear face 29 becomesserrated. Eventually, the rear face 29 disintegrates and forms droplets19 which acts as previously described in relation to the FIG. 7. Thesedroplets which become entrapped in the region 30 become metal inclusionsupon solidification and are defects which affect the operatingcharacteristics of the completed device.

For a more detailed description of the process and the apparatusemployed in the migration of selected dopant materials through a body ofsemiconductor material, including for example, the migration of aluminumin silicon, one is directed to our following copending patentapplications filed on the same day as this patent application andassigned to the same assignee as our present application: Method ofMaking Deep Diode Devices, Ser. No. 411,150 (U.S. Pat. No. 3,901,736);High Velocity Thermal Migration Method of Making Deep Diodes, Ser. No.411,021 (U.S. Pat. No. 3,901,801); Deep Diode Devices and Method andApparatus, Ser. No. 411,001 (now abandoned); High VelocityThermomigration Method of Making Deep Diodes, Ser. No. 411,015 (U.S.Pat. No. 3,898,106); Deep Diode Device and Method, Ser. No. 411,009(U.S. Pat. No. 3,902,925; and Stabilized Droplet Method of Making DeepDiodes Having Uniform Electrical Properties, Ser. No. 411,008 (U.S. Pat.No. 3,899,361).

Referring now to FIG. 9, a first electrical contact 32 is affixed to theregion 30 by a suitable ohmic solder layer 34. The large area powerdiode is completed by affixing a second electrical contact 36 to thebody 18 by a suitable layer 38 of ohmic solder material.

Large area power diodes prepared in the manner heretofore describedachieve excellent electrical characteristics without the necessity ofproviding passivating coatings on exposed surfaces of the devices,including the exposed surfaces of the P-N junction. The resulting P-Njunctions upon completion of processing are clean and very definitive.An abrupt step P-N junction is present. The breakdown voltage of diodesmade in accordance with the teachings of this invention achievessubstantially the theoretical breakdown voltage which is obtainable forthe semiconductor material of the bodies 10 and 18 employed and thedopant material comprising the layer 16. The leakage current of theprepareed diode devices is excellent when compared to prior art devices.

The following examples are illustrative of the teachings of thisinvention:

EXAMPLE I

Two wafers of silicon single crystal series conductor material, eachbeing one centimeter in thickness, were cut from a rod one inch indiameter. The silicon material of the rod was of N-type conductivitybeing phosphorus doped and being of 10 ohm-centimeters resistivity, 50microsecond lifetime, <111> crystallographic orientation with 1 × 10³dislocations per square centimeter. Each wafer was prepared in the usualmanner of polishing and lapping for semiconductor process work. A layerof aluminum metal 10 microns in thickness was deposited on one majorsurface of (111) planar orientation of one wafer by electron beam vapordeposition. The purity of the aluminum metal was 99.9999%. Deposition ofthe aluminum was practiced at a pressure of 1 × 10⁻⁵ torr and required50 minutes to deposit the layer of aluminum. No special preparation waspracticed to protect the sides and other major surfaces of the wafersince they were shielded by the type of apparatus employed.

The two wafers were then disposed relative to each other as shown inFIG. 2 to form the workpiece "sandwich". The sandwich was disposed inapparatus suitable for practicing thermal gradient zone melting. Theheat source was a molybdenum disk heated by an electron beam apparatus.The cold source, or heat sink, was a watercooled cooper disk. Acalculated temperature gradient of 100° C per centimeter was establishedin the silicon material of the semiconductor sandwich substantiallyaligned with the <111> axis of the wafers by heating the molybdenum diskto a temperature sufficient to heat the major surface of the waferclosest to the molybdenum disk to 1090° C ± 20° C and maintaining itthereat. The temperature of the water cooled copper disk was maintainedat 25° C ± 5° C. The process was practiced at a pressure of 1 × 10⁻⁵torr for 2 hours. The aluminum layer was not migrated entirely throughthe body of silicon.

Upon completion of the process, the sandwich was removed, sectioned, andexamined in infrared transmission. The two wafers had been physicallyjoined together. The P-N junction produced by the thermal gradient zonemelting process has an extremely sharp junction profile. Thealuminum-silicon migration had progressed a distance of 3 millimetersinto the wafer from the surface upon which the aluminum had beendeposited.

A sample diode 6.8 millimeters × 15 millimeters was cut from theprocessed sandwich and subjected to electrical testing. The P-N junctionarea was calculated to be 5 square millimeters. The breakdown voltage ofthe sample diode was 400 volts. This is the theoretical voltage whichcan be achieved for 10 ohm-centimeter N-type silicon doped with aluminumwherein the impurity concentration therein is approximately 2 × 10¹⁹atoms per cubic centimeter and the recrystallized semiconductor materialhas a resistivity of about 8 × 10⁻³ ohm-centimeter. The current leakageat 10 volts was 3 × 10⁻⁸ amperes. At 100 volts, the current leakage was1 × 10⁻⁷ amperes. The width of the P-N junction was determined bycapacitance versus voltage measurements. The P-N junction profile of theP-N junction formed by the initial aluminum layer and the uncoatedsilicon wafer was one-third of a micron in width. Very little diffusionof aluminum into silicon across the P-N junction had occurred during theprocess. The lifetime of the processed body "sandwich" material was onemicrosecond. This process was carried on under non-clean roomconditions.

A graph of the reverse current in amperes versus the reverse bias involts of a sample having a 3 square millimeter P-N junction area of theabove sample is shown in FIG. 10.

The diode as prepared has excellent electrical characteristics for boththe breakdown voltage and the current leakage requirements. As noted,these achievements were obtained without any special surface passivationmeans being employed for the processed example and without employingclean room conditions.

EXAMPLE II

The process of Example I was repeated except that the material of eachof the wafers had a <100> crystallographic orientation. The establishedthermal gradient was substantially aligned with the <100> axis of thewafers.

Examination of the P-N junction formed by the migrating interface showedthe interface to be highly irregular in shape. Such a device isunreliable for a large area high power diode device. However, low powerdiodes can be manufactured from the processed "sandwich" by trimmingback the sandwich to remove the irregular interface.

Large area power diodes prepared by the process of this inventionexhibit extremely sharp junction profiles. Very little diffusion of thealuminum into the silicon from across the P-N junction occurs in theprocess of this invention. The width of the P-N junction isapproximately one-third micron for a process temperature range of from700° C to approximately 1000° C. The lifetime of large area power diodesof this invention when prepared under clean room conditions are superiorto the lifetime of prior art devices.

An alternate method of making a high power diode embodies primarilyvapor deposition techniques for both the semiconductor material and itsdopant material. Referring now to FIG. 11, a body, or wafer, 110 ofsemiconductor material is prepared by suitable means such, for example,as by lapping and polishing for vapor deposition of metal thereon. Thebody 110 has a top surface 112 and a bottom surface 114. The dimensionsof the body 110 is as previously described for the bodies, or wafers, 10and 18. The material of the body 110 may be silicon, germanium, siliconcarbide, gallium arsenide, a compound of a Group II element and a GroupVI element and a compound of a Group III element and a Group V element.In order to describe the invention more specifically, the body 110 willbe described as being of silicon semiconductor material having N-typeconductivity.

A layer 116 of metal of about one micron to about 20 microns inthickness is deposited on the surface 112 by such suitable means, as byvapor deposition, electron beam, and the like, wherein the layer ofmetal deposited will be substantially oxygen and defect free. Thematerial of the layer 116 is one that will easily wet the surfaces ofthe semiconductor material that it comes in contact with during theinitial period of heating of the thermal gradient zone melting process.In addition, the material of the layer 116 comprises at least one metalsuitable for doping semiconductor material for producing a desiredP-type, N-type conductivity or intrinsic conductivity. A suitablematerial for comprising the layer 116 is aluminum when the body 110 issilicon of N-type conductivity.

A layer 118 of semiconductor material is deposited on the layer 116 ofthe metal. The semiconductor material may be the same as that of, or beany of the other materials suitable for comprising, the body 110. Thematerial of the layer 118 may be deposited by any suitable means such,for example, as by vapor phase deposition or by depositing the materialin powder form on the surface of the layer 116. The layer 118 shouldpreferably be of the same thickness as that thickness of the layer 116of metal to prevent the molten metal layer from penetrating the layer118 in one or more locations prior to the remainder of the molten layer.This prevents the possibility of having faulty devices resulting fromthe process. Since the thickness of the layer 118 is most usually small,in the order of up to 10 or 20 microns, orientation of thecrystallographic structure of the layer 118 is not critical. In thisexample, the layer 118 is said to be of polycrystalline silicon. on abody 18 of P-type silicon. In a like manner a recrystallized region 30of N⁺ -type conductivity is formed on a body 18 of N-type conductivity.

Referring now to FIG. 13, a semiconductor device 210 comprises threeregions 212, 214, and 216 of semiconductor materials having differentresistivities and the same or different type conductivity. For example,the region 212 may comprise N-type conductivity silicon, the region 214N⁺ -type conductivity silicon and the region 216 P⁺ -type conductivitysilicon. The interface 218 between the regions 214 and 216 is a P-Njunction formed by the contiguous surfaces of the regions 214 and 216 ofopposite type conductivity. The regions 214 and 216 are each formed fromrecrystallized bodies of semiconductor material of <111>crystallographic orientation like the body 10, and processed in the sameor similar manner as the body 10, to form the respective regions.Similarly, the device 210 may be of the configurations N--P⁺ --N⁺, P--P⁺--N⁺ and P--N⁺ --P⁺.

Similarly, the device 210 may be of a configuration wherein the region214 is of N-type conductivity and each of the regions 216 and 212 arerecrystallized bodies of semiconductor materials of P⁺ -typeconductivity and initially of <111> crystallographic structure. P-Njunctions are formed by the interfaces 218 and 220 of the respectivepairs of regions 214 and 216 and 214 and 212 of opposite typeconductivity. The regions 212 and 216 are formed by the thermal gradientzone melting process of this invention and the regions 212 and 216 arederived from bodies of semiconductor material prepared in the samemanner as the body 10 as hereintofore described. Other configurations ofthe device 210 prepared in this manner are as varied as thepossibilities of the conductivities of the regions 212, 214 and 216.Since each of the regions 212, 214 and 216 may each be of N-type, N⁺-type, P-type and P⁺ -type conductivity, the device 210 can be of any ofconfigurations that is possible by varying the conductivities of theregions 212, 214 and 216 in accordance with design requirements. Thesevarious configurations offer excellent opportunities for furtherprocessing of the device 210 into multi-region semiconductor devicessuch, for example, as power transistors, semiconductor controlledrectifiers, bidirectional switches, and the like. The choice ofsemiconductor material for the regions 212, 214 and 216 in accordancewith design requirements and the process requirements of this invention.

Alternately, the device 210 may be fabricated by starting with a body ofsuitable semiconductor material for the region 214. The regions 212 and216 may be obtained from vapor deposited layers of semiconductormaterial. Additionally, the region 216 or 212 may be obtained from vapordeposited semiconductor material. The other region 212 or 216 isobtained from a body of semiconductor material initially of <111>crystallographic structure.

The invention has been described relative to practicing thermal gradientzone melting in a negative atmosphere. However, it has been discoveredthat when each body of semiconductor material is a thin wafer of theorder of 10 mils in thickness, the thermal gradient zone melting processmay be practiced in an inert gas atmosphere of hydrogen, helium, argonand the like in a furnace having a positive atmosphere. The thickness ofthe wafers, or bodies, may also exceed 10 mils provided thecrystallographic orientation of the material is maintained for thematerial through which migration is practiced.

We claim as our invention:
 1. A process for making a semiconductordevice comprising:a. selecting a first body of single crystal siliconsemiconductor material having top and bottom surfaces which are opposedmajor surfaces thereof and at least the top surface has a preferredcrystal orientation corresponding to the natural solid-liquid facetplane thereof; b. vapor depositing a first layer of metal comprising atleast aluminum of a preferred thickness on the top surface of the firstbody and in intimate contact therewith; c. placing the first body in anabutting contact relationship with a second body of single crystalsilicon semiconductor material having top and bottom surfaces which areopposed major surfaces thereof wherein the layer of metal on the topsurface on the first body is in an abutting contact relationship withthe top surface of the second body; d. heating the two bodies and thelayer of metal to an elevated temperature sufficient to form a moltenregion of the metal of the layer and the semiconductor material of thetwo bodies in contact therewith, the molten region having a preferredthickness not greater than the value of L which is determined from thefollowing relationship ##EQU5## whereL is the thickness of the moltenregion in centimeters, V is the velocity of the molten zone through thesolid semiconductor material in centimeters per second, (dc/dt) is thechange in concentration of silicon in the molten zone with respect tochange in temperature in atomic % of silicon per degree celsius, andG_(l) is the temperature gradient across the thickness of molten regionin degrees celsius per centimeter; e. establishing a thermal gradientsubstantially along a preferred crystallographic axis of the first bodywhich is normal to the natural solid-liquid facet plane of thesemiconductor metal-liquid surface wherein the bottom surface of thefirst body is at the highest temperature; f. migrating the molten zonethrough the first body from the top to the bottom surfaces substantiallyalong the preferred axis to form a first region of recrystallizedmaterial of a selected portion of the second body having solidsolubility of the metal of the layer therein of the second body and toform a second region of recrystallized material of the second bodyhaving solid solubility of the metal of the layer therein, the solidsolubility metal imparting a selected type conductivity and a selectedlevel of resistivity to the two regions, the first and second regionsbeing integral with each other, substantially free of metal inclusions,and having the crystalline structure of the second body, and g. joiningtogether the two bodies of semiconductor material by the integralregions of recrystallized semiconductor material.
 2. The process ofclaim 1 whereinthe preferred thickness of the layer of metal is at leastone micron and less than about 20 microns.
 3. The process of claim 2whereinthe preferred crystal orientation of the natural solid-liquidfacet plane is (111), and the preferred crystallographic axis of thefirst body is <111>.
 4. The process of claim 2 whereinthe temperaturegradient is from approximately 50° C/cm to approximately 200° C/cm. 5.The process of claim 4 whereinthe elevated temperature is fromapproximately 700° C to about 1400° C.
 6. The process of claim 1whereinthe preferred crystal orientation of the natural solid-liquidfacet plane is (111), and the preferred crystallographic axis of thefirst body is <111>.
 7. The process of claim 6 whereinthe temperaturegradient is from approximately 50° C/cm to approximately 200° C/cm. 8.The process of claim 7 whereinthe temperature gradient is approximately100° C/cm.
 9. The process of claim 8 whereinthe elevated temperature is1000° C ± 20° C.
 10. The process of claim 7 whereinthe semiconductormaterial of the second body is of N-type conductivity, and wherein theregions of recrystallized semiconductor material having P-typeconductivity, and including the process step following the migrating ofthe molten zone through the first body of forming a P-N junction by thecontiguous surfaces of the material of the second body and the integralregions of recrystallized semiconductor material.
 11. The process ofclaim 10 whereinthe resistivity of the two bodies prior to theinitiation of migration is 10 ohm-centimeter, and the resistivity of theregion of recrystallized semiconductor material is approximately 8 ×10⁻³ ohm-centimeter.
 12. The process of claim 7 whereinthe elevatedtemperature is from approximately 700° C to about 1400° C.
 13. Theprocess for making the semiconductor device of claim 1 including theadditional process steps of:h. selecting a third wafer of single crystalsilicon semiconductor material having top and bottom surfaces which areopposed major surfaces thereof and at least the top surface has apreferred crystal orientation corresponding to the natural solid-liquidfacet plane thereof; i. vapor depositing a second layer of metalcomprising at least aluminum of a preferred thickness on the top surfaceof the third body and in intimate contact therewith, j. placing thethird body in an abutting contact relationship with the second body ofsingle crystal silicon semiconductor material wherein the layer of metalon the top surface of the third body is in an abutting contactrelationship with the bottom surface of the second body; k. heating thebodies and the second layer of metal to an elevated temperaturesufficient to form a molten region of the metal of the second layer andthe semiconductor material of the second and third bodies in contacttherewith, the molten region having a preferred thickness not greaterthan the value of L which is determined as before in process step (d) l.establishing a thermal gradient substantially along a preferredcrystallographic axis of the third body which is normal to the naturalsolid-liquid face plane of the semiconductor metal-liquid surfacewherein the bottom surface of the third body is at the highesttemperature; m. migrating the molten zone through the third bodysubstantially along the preferred axis of the third body to form a thirdregion of recrystallized material of a second selected portion of thesecond body having solid solubility of the metal of the layer therein ofthe second body and to form a fourth region of recrystallized materialof the third body having solid solubility metal imparting a selectedtype conductivity and a selected level of resistivity to the third andfourth regions, the third and fourth regions being integral with eachother, substantially free of metal inclusions, and having thecrystalline structure of the second body, and n. joining together thetwo bodies of semiconductor material by the integral third and fourthregions of recrystallized semiconductor material.
 14. The process ofclaim 13 whereinthe preferred crystal orientation of the naturalsolid-liquid facet plane is (111), and the preferred crystallographicaxis of the third body is < 111 >.
 15. The process of claim 13whereinthe preferred thickness of the layer of metal is at least onemicron and less than about 20 microns.
 16. The process of claim 15whereinthe preferred crystal orientation of the natural solid-liquidfacet plane is (111), and the preferred crystallograpic axis of thethird body is <111 >.
 17. The process of claim 16 whereinthe temperaturegradient is from approximately 50° C/cm to approximately 200° C/cm. 18.The process of claim 17 whereinthe semiconductor material of the secondbody is of N-type conductivity wherein the third and fourth regions ofrecrystallized semiconductor material have P-type conductivity, andincluding the process steps following the migrating of the molten zonethrough the third body of forming a P-N junction by the contiguoussurfaces of the material of the second body and the integral third andfourth regions of recrystallized semiconductor material.
 19. The processof claim 17 whereinthe elevated temperature is from approximately 700° Cto about 1400° C.
 20. The process of claim 15 whereinthe temperaturegradient is from approximately 50° C/cm to approximately 200° C/cm. 21.The process of claim 20 whereinthe elevated temperature is fromapproximately 700° C to about 1400° C.